The invention relates to a system for transmitting digital signals by radio between a plurality of subscriber stations and a base station using a multicarrier code division multiple access method.
In many radio transmission systems, particular in modern mobile radio systems, the allocated frequency bands have to be used as efficiently as possible because of the restricted number of frequencies available. Furthermore, radio transmission systems, in particular of the mobile kind, must be configured in such a way that mutual interference is so minimal that they can be operated one next to the other. In particular in mobile radio systems there are two further relatively serious difficulties, namely on the one hand the problem of multiple access (MA), which arises owing to the simultaneous transmission of a plurality of signals which are assigned to subscribers which are respectively active and which use the same RF carrier, and on the other hand the equalization problem which arises owing to the frequency-selectivity of the radio channel. Code Division Multiple Access, also referred to as CDMA, is a known and convenient solution of the multiple access problem. In CDMA radio systems, a plurality of subscribers transmit their subscriber signals in a common RF frequency band over a time-variant and frequency-selective radio channel. However, time-variant mutual interference, which is referred to as Multiple Access Interference (MAI) may arise between the simultaneously transmitted signals and can be reduced by means of suitable signal separation techniques. In addition, in CDMA radio systems, time-variant Intersymbol Interference (ISI) may arise between the data symbols which are successively transmitted by one specific subscriber. The signal mixture arising at the receiver can be evaluated by means of individual subscriber detection or advantageously separated by means of algorithms for multisubscriber detection. This separation is advantageous in terms of the implementation of CDMA systems in particular in terrestrial mobile radio because it is possible to dispense with complex methods for power control and for soft handover. In addition, diversity reception, for example as a result of the use of a plurality of receiver antennas (antenna diversity) is advantageous since it enables the transmission quality to be improved. CDMA systems with direct code spread (DS, Direct Sequence), diversity reception and multisubscriber detection are known. An advantageous method which is applied successfully in DS-CDMA systems for multisubscriber detection is the so-called JD (Joint Detection) method which is described, for example, in the essay by P. Jung, B. Steiner: "Konzept eines CDMA-Mobilfunksystems mit gemeinsamer Detektion fur die dritte Mobilfunkgeneration [Concept of a CDMA mobile radio system with common detection for the third mobile radio generation]", Parts 1 and 2, "Nachrichtentech., Elektron. [Telecommunications, electronics]", SCIENCE, Berlin 45 (1995) 1, pages 10 to 14 and 2, pages 24 to 27. An important advantage of such CDMA systems is the utilization of frequency diversity and interference diversity. A disadvantage with DS-CDMA systems is the low degree of influence on the dividing up and allocation of the frequency resource. Combining CDMA systems with multicarrier (MC) methods eliminates this disadvantage.
Multicarrier transmission methods have their origin in the orthogonal frequency division multiplexing (OFDM) technique. In OFDM, the carrier bandwidth B.sub.u which is assigned to a particular subscriber k is divided up into Q.sub.T subcarriers, lying one next to the other, with identical bandwidth B.sub.s. This results in: EQU B.sub.U =Q.sub.T.multidot.B.sub.s (1)
In order to permit overlapping subcarriers which are orthogonal with respect to the data symbol period T.sub.s, the bandwidth B.sub.s equal to 1/T.sub.s is selected. Orthogonal subcarriers facilitate the use of receivers of simple design. Below, it is assumed that k subscribers transmit N, to the base m, complex data symbols d.sub.n (K), n=1 . . . N within the time period T.sub.bu. The data symbols d.sub.n.sup.(k), n=1 . . . N are taken from the complex set EQU V={v.sub.1,v.sub.2 . . . v.sub.m }, v.sub..mu..epsilon.C, .mu.=1 . . . m, m.epsilon.IN (2).
The transmission takes place via the RF carrier with the bandwidth B.sub.u specified in equation (1). With OFDM, the following applies: EQU Q.sub.T =N (3).
Each data symbol d.sub.n.sup.(k) is assigned here to a particular subcarrier. For this reason, all the data symbols d.sub.n.sup.(k) are transmitted simultaneously during the previously mentioned time period T.sub.bu. The symbol period T.sub.s is thus equal to the time period T.sub.bu in OFDM.
The energy of MC-CDMA signals is restricted in spectral terms very well to the assigned RF carrier with the bandwidth B.sub.u, which is due to the closeness to OFDM. Thus, adjacent channel interference is very low. This fact is very advantageous in terms of the coexistence of the systems. Furthermore, the spectra of MC-CDMA signals in the band B.sub.u are rather white, which is favorable in terms of equalization and detection. The fading phenomena experienced by each subcarrier are largely frequency-unselective because the bandwidth of each subcarrier B.sub.s is usually narrower than the coherent bandwidth B.sub.c of a mobile radio channel. This absence of frequency-selectivity in conjunction with the orthogonality of the subcarriers make it possible to use simple suboptimum detectors in MC-CDMA. Since a data symbol d.sub.n.sup.(k) is assigned to a single subcarrier in OFDM, OFDM has a low frequency diversity capacity. On the other hand, in MC-CDMA, a data symbol d.sub.n.sup.(k) is transmitted over Q subcarriers simultaneously where 1&lt;Q.ltoreq.Q.sub.T, which permits good utilization of frequency diversity. If frequency gaps are inserted between the Q subcarriers to which a particular data symbol d.sub.n.sup.(k) is allocated, the frequency diversity behavior can easily be brought about, which additionally increases the flexibility of the system. For example, subcarriers assigned to other data symbols d.sub.n.sup.(k), n'.apprxeq.n can be assigned in the frequency gaps between the previously mentioned Q subcarriers. The technique of frequency diversity is not considered further below. Instead, it is assumed that all the Q subcarriers assigned to a specific data symbol d.sub.n.sup.(k) are adjacent, which still permits frequency diversity to be utilized provided provision is made to ensure that Q.multidot.B.sub.s exceeds the coherence bandwidth B.sub.c. MC-CDMA is also beneficial with interference diversity because K&gt;1 subscribers actively use the same Q subcarriers simultaneously. Interference diversity is the key feature in achieving a high spectral efficiency .eta..
Existing system concepts for CDMA systems with multicarrier (MC) methods are not suitable for general use in mobile radio. Structures for CDMA systems with multicarrier (MC) methods have existed hitherto only for environments with low time variance and negligible intersymbol interference. Algorithms for multisubscriber detection have hitherto been proposed and investigated only for the downlink (from the base station to the subscriber stations). Until now, only conventional individual subscriber detectors have been proposed and investigated for the uplink (from the subscriber station to the base station) which is rarely considered because it is more complex. Diversity reception, for example Coherent Receiver Antenna Diversity (CRAD) in such CDMA systems with multicarrier methods has not been investigated to date.
The relation between the number Q.sub.T of all the subcarriers on the RF frequency band B.sub.u and the number Q of the subcarriers assigned to a particular data symbol d.sub.n.sup.(k) is as follows in MC-CDMA: EQU Q.sub.T =Q.multidot.N.sub.s (4).
In the equation (4), N.sub.s signifies the number of data symbols d.sub.n.sup.(k) transmitted simultaneously by a subscriber k. The data symbol period length is thus ##EQU1##
For given values of B.sub.u and T.sub.bu, a MC-CDMA system concept depends on the selection of Q and N.sub.s.
Different selections of Q and N.sub.s lead to possible MC-CDMA concepts with particular features. Given a known possible MC-CDMA concept, N.sub.s =N, which means that all the data symbols d.sub.n.sup.(k) are transmitted simultaneously. In addition, with this known concept B.sub.s =B.sub.u /(Q.multidot.N) and T.sub.s =T.sub.bu apply. According to a series of investigations of MC-CDMA, this known concept facilitates the advantageous avoidance of time-variable intersymbol interference by introducing protective intervals of the period length T.sub.g &gt;T.sub.M (T.sub.M =duration of the channel pulse response) However, the introduction of protective intervals can be used only to a certain extent if T.sub.s &gt;&gt;T.sub.g. In mobile radio environments, the length T.sub.M of the channel pulse response is in the order of magnitude of between several microseconds and several tens of microseconds, which requires a data symbol period length of T.sub.s &gt;100 .mu.s.